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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical
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Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical

机译：TEMPO自由基激发萘光物理猝灭过程中电子自旋极化的距离依赖性

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摘要

Quenching of excited states by a free radical is generally studied in systems where these two are separate entities freely moving in a liquid solution. Random diffusive encounters bring them together to cause the quenching and leave the spins of the radical polarized. In the dynamics of the radical-triplet pair mechanism of the generation of electron spin polarization (ESP), the distance-dependent exchange interaction plays a crucial role. To investigate how the distance between the excited molecule and the radical influences the ESP, we have covalently linked a naphthalene moiety to a TEMPO free radical through a spacer group of three different lengths. We compared the ESP process of these linked compounds with that of the usual "unlinked system" of naphthalene and TEMPO through time-resolved EPR experiments at low temperature in n-hexane solution. The time evolution of both the linked and the "unlinked system" was treated on a similar footing. The time-dependent EPR signal was analyzed by combining photophysical kinetics and time-dependent Bloch equations incorporating spin dynamics. Sequential quenching of the singlet state and the triplet state of naphthalene was seen in all the systems, as revealed through the spin-polarized TREPR spectra of opposite phase. The magnitudes of the ESP in the linked molecules were higher than those of the "unlinked system," showing that when the two moieties are held together greater mixing of quartet-doublet states takes place. The magnitudes of ESP steadily decrease with increasing the length of the spacer group. The polarization magnitudes due to triplet quenching and singlet quenching are very similar, differing by a factor of only similar to 2. These characteristics show that for all the linked molecules the quenching takes place in the "weak exchange" regime and at almost the same distance of separation between the two moieties. Our results also showed that observation of small absorptive TREPR signals does not necessarily imply that its magnitude of polarization is small.
机译：通常在其中这两个是在液体溶液中自由移动的独立实体的系统中研究自由基对激发态的淬灭。随机的扩散相遇将它们聚集在一起以引起猝灭，并使自由基的自旋极化。在电子自旋极化（ESP）产生的自由基-三重态对机制的动力学中，距离相关的交换相互作用起着至关重要的作用。为了研究激发分子与自由基之间的距离如何影响ESP，我们通过三个不同长度的间隔基团将萘部分与TEMPO自由基共价连接。通过在低温下于正己烷溶液中进行时间分辨的EPR实验，我们将这些连接的化合物的ESP工艺与常规的萘和TEMPO“未连接系统”的ESP工艺进行了比较。链接系统和“非链接系统”的时间演变都以类似的方式处理。通过结合光物理动力学和结合自旋动力学的时间依赖性Bloch方程来分析时间依赖性EPR信号。通过反相的自旋极化TREPR光谱显示，在所有系统中都可以看到萘的单重态和三重态的顺序淬灭。连接分子中ESP的幅度高于“未连接系统”中的ESP幅度，表明当两个部分保持在一起时，发生了更多的四重双峰态混合。 ESP的幅度随着间隔基团长度的增加而稳定降低。三重态猝灭和单重态猝灭引起的极化幅度非常相似，相差仅2倍。这些特征表明，对于所有连接的分子，猝灭均发生在“弱交换”状态且几乎相同的距离两个部分之间的分离。我们的结果还表明，观察到小的吸收性TREPR信号并不一定意味着其极化幅度很小。

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